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TPS92520-Q1: Synchronous mosfet and 16:1 dimming range

Dustin Yim
Intellectual 2140 points
Part Number: TPS92520-Q1

Hi Experts,

I have poor knowledge about synchronous FET Dimmig.

I don't understand about "dimming range of 16:1".

Could you please explain about this?

In data sheet,

7.3.6 Analog Dimming and Forced Continuous Conduction Mode

Analog dimming is accomplished by the SPI interface through the adjustment of the 10-bit CHxIADJ registers.

The TPS92520-Q1 improves the linear range of analog dimming by supporting forced continuous conduction mode of operation.

With synchronous MOSFETs, the inductor current is allowed to go negative for part of the switching cycle,

thus enabling linear dimming with over 16:1 dimming range.

Thank you.

  • 0
    TI__Genius 17205 points

    The dimming ratio is the ratio between the highest achievable brightness of the LED, which is controlled by the current, and lowest  attainable brightness level and that brightness range needs to be linear between min and max brightness levels.  For the 520 the "Analog" dimmring ratio is a function of the max and min current, where we can go up to 1.6A and we state that we can support 0.1A for a low, thus giving 1.6/0.1 = 16:1.  In actuality, we can go much higher ratios because it is a synchronous buck part and we can achieve much lower currents while still providing linearity.  A non-synchronous buck would have discontinuous currents at light load because it can't support negative currents of the ripple.  That means it would not be linear at lower current setpoints compared to the higher current setpoints.  

    Here is a link to the video that shows this information.  

    https://training.ti.com/dimming-methods-analog-dimming 

    Keep in mind we can do much higher ratios when doing PWM dimming and shunt fet dimming.  

    -fhoude

  • 0 in reply to fhoude
    TI__Intellectual 2140 points

    Thanks for your reply.

    "Keep in mind we can do much higher ratios when doing PWM dimming and shunt fet dimming. "

    => Why we can do higher ratios when using shunt fet dimming?

    As far as I know, Shunt fet dimming is simmilar with LED driver Matrix. But it is passive operation.

  • 0 in reply to Dustin Yim
    TI__Genius 17205 points

    Hello Dustin,

    You don't loose the accuracy of the output at light loads when doing PWM dimming or shunt fet dimming.  You can control average current from 100% down to under 1% when doing PWM/shunt fet dimming.  You can't do that with analog dimming of a buck that is not synchronous. 

    Yes, Matrix is essentially shunt fet dimming.  Not sure what you mean by "passive operation".